at.html

unix 下的C开发手册,还用详细的例程。

%token year_number
/*
  A year_number is a four-digit number representing the year A.D., in
  which the at_job is to be run.
*/

%token inc_number
/*
  The inc_number is the number of times the succeeding increment
  period is to be added to the specified date and time.
*/

%token timezone_name
/*
  The name of an optional timezone suffix to the time field, in an
  implementation-dependent format.
*/

%token month_name
/*
  One of the values from the "mon" or "abmon" keywords in the LC_TIME
  locale category.
*/

%token day_of_week
/*
  One of the values from the "day" or "abday" keywords in the LC_TIME
  locale category.
*/

%token am_pm
/*
  One of the values from the "am_pm" keyword in the LC_TIME locale
  category.
*/

%start timespec
%%
timespec    : time
            | time date
            | time increment
            | time date increment
            | nowspec
            ;

nowspec     : "now"
            | "now" increment
            ;

time        : hr24clock_hr_min
            | hr24clock_hr_min timezone_name
            | hr24clock_hour ":" minute
            | hr24clock_hour ":" minute timezone_name
            | wallclock_hr_min am_pm
            | wallclock_hr_min am_pm timezone_name
            | wallclock_hour ":" minute am_pm
            | wallclock_hour ":" minute am_pm timezone_name
            | "noon"
            | "midnight"
            ;

date        : month_name day_number
            | month_name day_number "," year_number
            | day_of_week
            | "today"
            | "tomorrow"
            ;

increment   : "+" inc_number inc_period
            | "next" inc_period
            ;

inc_period  : "minute" | "minutes"
            | "hour" | "hours"
            | "day" | "days"
            | "week" | "weeks"
            | "month" | "months"
            | "year" | "years"
            ;
</code>
</pre>
</blockquote><h4><a name = "tag_000_000_082">&nbsp;</a>STDIN</h4><blockquote>
The standard input must be
a text file consisting of commands acceptable to the
shell command language described in
<xref href=shell><a href="chap2.html#tag_001">
Shell Command Language
</a></xref>.
The standard input will only be used if no
<b>-f</b>&nbsp;<i>file</i>
option is specified.
</blockquote><h4><a name = "tag_000_000_083">&nbsp;</a>INPUT FILES</h4><blockquote>
See the STDIN section.
<p>
The text files
<b>/usr/lib/cron/at.allow</b>
and
<b>/usr/lib/cron/at.deny</b>
contain user names, one per line, of users who are, respectively,
authorised or denied access to the
<i>at</i>
and
<i><a href="batch.html">batch</a></i>
utilities.
</blockquote><h4><a name = "tag_000_000_084">&nbsp;</a>ENVIRONMENT VARIABLES</h4><blockquote>
The following environment variables affect the execution of
<i>at</i>:
<dl compact>

<dt><i>LANG</i><dd>Provide a default value for the internationalisation variables
that are unset or null.
If
<i>LANG</i>
is unset or null, the corresponding value from the
implementation-dependent default locale will be used.
If any of the internationalisation variables contains an invalid setting, the
utility will behave as if none of the variables had been defined.

<dt><i>LC_ALL</i><dd>
If set to a non-empty string value,
override the values of all the other internationalisation variables.

<dt><i>LC_CTYPE</i><dd>
Determine the
locale for the interpretation of sequences of bytes of text data as
characters (for example, single- as opposed to multi-byte characters
in arguments and input files).

<dt><i>LC_MESSAGES</i><dd>
Determine the locale that should be used to affect
the format and contents of diagnostic
messages written to standard error
and informative messages written to standard output.

<dt><i>NLSPATH</i><dd>
Determine the location of message catalogues
for the processing of
<i>LC_MESSAGES .
</i>
<dt><i>LC_TIME</i><dd>
Determine the format and contents for date and time
strings written and accepted by
<i>at</i>.

<dt><i>SHELL</i><dd>Determine a name
of a command interpreter to be used to invoke the at-job.
If the variable is unset or null,
<i><a href="sh.html">sh</a></i>
will be used.
If it is set to a value other than a name for
<i><a href="sh.html">sh</a></i>,
the implementation will do one of the following:
use that shell;
use
<i><a href="sh.html">sh</a></i>;
use the login shell from the user database; or
any of the preceding accompanied by a
warning diagnostic about which was chosen.

<dt><i>TZ</i><dd>Determine the timezone.
The job will be submitted for execution at the
time specified by
<i>timespec</i>
or
<b>-t</b>&nbsp;<i>time</i>
relative to the timezone specified by the
<i>TZ</i>
variable.
If
<i>timespec</i>
specifies a timezone, it will override
<i>TZ .
</i>If
<i>timespec</i>
does not specify a timezone and
<i>TZ</i>
is unset or null, an unspecified default timezone will be used.

</dl>
</blockquote><h4><a name = "tag_000_000_085">&nbsp;</a>ASYNCHRONOUS EVENTS</h4><blockquote>
Default.
</blockquote><h4><a name = "tag_000_000_086">&nbsp;</a>STDOUT</h4><blockquote>
When standard input is a terminal, prompts of unspecified
format for each line of the user input described in
the STDIN section may be written to standard output.
<p>
In the POSIX locale,
the following will be written to the standard output
for each job when jobs are listed in response to the
<b>-l</b>
option:
<code>
<p>
<tt>"%s\t%s\n"</tt>, <i>at_job_id</i>,
&lt;<i>date</i>&gt;
</code>
where
&lt;<i>date</i>&gt;
is equivalent in format to the output of:
<pre>
<code>
date +"%a %b %e %T %Y"
</code>
</pre>
<p>
The date and time written will be adjusted so
that they appear in the timezone of the user (as
determined by the
<i>TZ</i>
variable).
</blockquote><h4><a name = "tag_000_000_087">&nbsp;</a>STDERR</h4><blockquote>
The following will be written to standard error
when a job has been successfully submitted:
<code>
<p>
<tt>"job %s at %s\n"</tt>, <i>at_job_id</i>,
&lt;<i>date</i>&gt;
</code>
where
&lt;<i>date</i>&gt;
has the same format as is described in the STDOUT section.
Neither this, nor warning messages concerning the selection
of the command interpreter, are considered a diagnostic
that changes the exit status.
<p>
Diagnostic messages, if any, are written to standard error.
</blockquote><h4><a name = "tag_000_000_088">&nbsp;</a>OUTPUT FILES</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_000_089">&nbsp;</a>EXTENDED DESCRIPTION</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_000_090">&nbsp;</a>EXIT STATUS</h4><blockquote>
The following exit values are returned:
<dl compact>

<dt>0<dd>The
<i>at</i>
utility successfully submitted, removed or listed
a job or jobs.

<dt>&gt;0<dd>An error occurred.

</dl>
</blockquote><h4><a name = "tag_000_000_091">&nbsp;</a>CONSEQUENCES OF ERRORS</h4><blockquote>
The job will not be scheduled, removed or listed.
</blockquote><h4><a name = "tag_000_000_092">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
The format of the
<i>at</i>
command line shown here is guaranteed only for the POSIX locale.
Other cultures may be supported with substantially different
interfaces, although implementations are encouraged to provide
comparable levels of functionality.
<p>
Since the commands run in a separate shell invocation,
running in a separate process group
with no controlling terminal,
open file descriptors, traps and priority
inherited from the invoking environment are lost.
<p>
Some implementations do not allow substitution of
different shells using
<i>SHELL .
</i>System&nbsp;V systems, for example, have used the
login shell value for the user in
<b>/etc/passwd</b>.
To select reliably another command interpreter,
the user must include it as part of the script, such as:
<pre>
<code>
<b>$</b> at 1800
myshell myscript
<b>job ... at ...
$
</b></code>
</pre>
</blockquote><h4><a name = "tag_000_000_093">&nbsp;</a>EXAMPLES</h4><blockquote>
<ol>
<p>
<li>
This sequence can be used at a terminal:
<pre>
<code>
at -m 0730 tomorrow
sort &lt; file &gt;outfile
EOT
</code>
</pre>
<p>
<li>
This sequence, which demonstrates redirecting standard
error to a pipe, is useful in a command procedure (the sequence of
output redirection specifications is significant):
<pre>
<code>
at now + 1 hour &lt;&lt;!
diff file1 file2 2&gt;&amp;1 &gt;outfile | mailx mygroup
!
</code>
</pre>
<p>
<li>
To have a job reschedule itself,
<i>at</i>
can be invoked from within the
at-job.
For example, this daily processing script named
<b>my.daily</b>
will run every day (although
<i><a href="crontab.html">crontab</a></i>
is a more appropriate vehicle for such work):
<pre>
<code>
# my.daily runs every day
<i>daily processing</i>
at now tomorrow &lt; my.daily
</code>
</pre>
<p>
<li>
The spacing of the three portions of the
POSIX locale
<i>timespec</i>
is quite flexible
as long as there are no ambiguities.
Examples of various times and operand presentation include:
<pre>
<code>
at 0815am Jan 24
at 8 :15amjan24
at now "+ 1day"
at 5 pm FRIday
at '17
        utc+
        30minutes'
</code>
</pre>
<p>
</ol>
</blockquote><h4><a name = "tag_000_000_094">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_000_095">&nbsp;</a>SEE ALSO</h4><blockquote>
<i><a href="batch.html">batch</a></i>,
<i><a href="crontab.html">crontab</a></i>.
</blockquote><hr size=2 noshade>
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